Polymeric films are used in a wide variety of applications. One particular use of polymeric films is in mirrors which reflect light over a particular wavelength range. Such reflective films can be disposed, for example, behind a backlight in liquid crystal displays to reflect light toward the display to enhance brightness of the display. Color shifting films can be used in signage, packaging materials, etc. IR mirror films can be used, for example, to reduce solar heat load entering a building or vehicle through its windows. Ultraviolet (UV) films can be used to protect other films or objects from UV light to prevent deleterious effects (e.g., photodegradation of a polymeric film).
Coextrusion casting processes have been used to make multilayer optical mirrors. Generally, however, cast films have a number of practical drawbacks. For example, cast films generally have low refractive index differences between the high and low index materials and do not generally have matching refractive indices in the z-direction, limiting the optical performance for a given number of layers. Because of the limited optical power of such cast films, dyes and pigments also typically are used to enhance the color of color mirror films. Moreover, some cast films, particularly films made of noncrystalline materials, can also have limited thermal stability, dimensional stability, environmental stability and/or solvent resistance.
Coextrusion-orientation processes have been used to provide films with better optical performance due to the large refractive index difference between high and low index materials and the capability of matching refractive indices in the z-, or out-of-plane direction when at least one of the materials is birefringent. One example of a previously formed film has high index layers formed of polyethylene naphthalate (PEN) and low index layers of polymethyl methacrylate (PMMA). Orientation of PEN increases the refractive indices of the PEN layers and, therefore, increases the optical power of the PEN/PMMA films. PEN, however, is a relatively expensive material which is difficult to protect from ultraviolet radiation, and polyethylene terephthalate (PET), a lower index alternative to PEN, cannot easily be suitably oriented with PMMA due to the difference in glass transition temperatures of these materials (about 84° C. for PET and about 106° C. for PMMA).